The PON becomes a perfect optical access technology because of its advantages of being easy to maintain, a high bandwidth, a low cost, etc., which is an ideal physical platform to which various services of voice, data, video, etc., gain an integrated access through a single platform. The PON technology is an optical fiber access technology of point-to-multipoint (P2MP). The PON including an optical line terminal (OLT), an optical network unit (ONU) and an optical distribution network (ODN) takes advantage of a passive splitter/coupler in the ODN to dispense with an element capable of amplification and relaying.
Due to the use of a point-to-multipoint topology for the PON, a point-to-multipoint multiple access protocol has to be adopted to enable numerous ONUs to share the OLT and an optical fiber backbone. As stipulated in the PON system, the direction of data from the OLT to the ONU is referred to as the downlink direction and the direction from the ONU to the OLT is referred to as the uplink direction. Currently, the widely applied uplink and downlink transmission modes of the PON system involve the use of a broadcast mode of time division multiplexing (TDM) in the PON downlink and the use of an access mode of time division multiple access (TDMA) in the uplink.
Unlike the conventional point-to-point consecutive communication mode, a multipoint-to-point burst communication mode is adopted in the PON uplink. The uplink channel is shared in the TDMA access mode for PON uplink transmission. The OLT can allocate different time slots to the respective ONUs, each of which can only transmit its own data information block in the timeslot assigned by the OLT.
Because the different ONUs are at different distances from the OLT end in the PON system, signals received at the OLT end from the different ONUs also have different intensities. Therefore upon reception at the OLT of a burst data frame from the ONU, the OLT reception end has to perform an automatic gain control (AGC) and a clock data recovery (CDR) by utilizing a synchronization pattern (a preamble) in the received burst frame, and then the OLT can matches a burst delimiter (BD) with the received burst frame and know from the match the starting location of data in the received burst frame to thereby receive the data.
A burst frame transmitted in the uplink of the existing PON system is structured in a way that a binary sequence of “1010 . . . ” (a hexadecimal sequence of “0x55 . . . ”) with alternating zeros and ones is defined as a current synchronization pattern which is used for the OLT to perform the AGC and the clock recovery on a received burst frame. It was identified in practice that the spectrum of the synchronization pattern signal is concentrated at high frequency components, which might be adverse to the use of a less complex equalizer at the OLT reception end. Furthermore, a frequently transition may result in the incapability of an existing peak detector to detect an actual peak of a received signal and hence possibly in degraded sensitivity of a receiver.